1. Field of the Invention
The present invention generally relates to a device having a capacitor, such as an oscillator, and more particularly, to an interdigital capacitor used in a high-frequency circuit.
2. Description of the Related Art
An MIM (Metal Insulator Metal) capacitor and an interdigital capacitor are known as capacitors used in high-frequency circuits that handle a frequency as high as 1 GHz or over. Usually, these capacitors are incorporated in an MMIC (Monolithic Microwave Integrated Circuit). For example, the MIM is made up of a pair of electrodes formed on a semi-insulating GaAs substrate, and a dielectric member interposed between the pair of electrodes. The interdigital capacitor has an interdigital electrode pattern formed on a semi-insulating GaAs substrate.
Generally, the capacitor is required to have a smaller capacitance, as the frequency is higher. For example, the capacitance required in the 40 GHz band is as small as 30 fF. The capacitor having such a small capacitance should be compact. For instance, a capacitance of 30 fF by an MIM capacitor having a per-unit-area capacitance of 0.4 fF/μm2 needs an extremely small squire electrode having a side of 8.7 μm. The capacitance will deviate from the capacitance of 30 fF unless the above dimensions are accurately realized. Such a deviation of the capacitance will affect the circuit operation. For example, the oscillator employing the MIM capacitor will have a frequency error that depends on a capacitance deviation. It is to be noted that current technology has a difficulty in accurate production of an electrode pattern of MIM capacitor having a side equal to or smaller than 10 μm. In other words, the MIM capacitors currently available have a large variation in capacitance.
In contrast, the interdigital capacitor is suitable for use in a millimeter wave band higher than 30 GHz, and is capable of accurately realizing a capacitance as small as tens of fF. FIG. 1 illustrates an interdigital capacitor, which is made up of a semiconductor substrate of, for example, silicon or a semi-insulating GaAs, and an interdigital electrode pattern formed on the substrate. The electrode pattern is composed of two comb-like electrodes 10 and 12. Electrode fingers of the comb-like electrodes 10 and 12 are alternately arranged. In order to realize a capacitance of about 30 fF by the interdigital capacitor, it should have an electrode finger width of 5 μm, an finger-to-finger gap of about 2 μm, and dimensions of 50 μm×50 μm as a whole.
As shown in FIG. 2, the interdigital capacitor may be used as a decoupling capacitor that is provided between high-frequency amplifiers AMP1 and AMP2. The amplifiers AMP1 and AMP2 and the interdigital capacitor are approximately aligned. This arrangement is little affected by parasitic inductance coupled to interconnection or wiring lines. In contrast, a problem may occur for a circuit made up of a transistor and some interdigital capacitors connected thereto. An example of such a circuit is an oscillator. In many cases, it is difficult to connect the interdigital capacitors and the transistors in a short distance. As the interconnection length increases, the parasitic inductance increases. An increased parasitic inductance may affect the circuit operation. High-frequency circuits are required to have symmetry of circuit pattern. For instance, in case where a differential circuit fails to have a symmetrical circuit pattern, a phase difference between differential signals may arise therefrom. The phase difference serves as a noise source. It is to be noted that there is difficulty in designing a symmetrical circuit pattern on the substrate in which some transistors and the interdigital capacitor shown in FIG. 1 are connected with short interconnection lines. Problems that arise from parasitic inductance and asymmetry of circuit pattern greatly affect circuit operations and characteristics, particularly, in a millimeter wave range over 30 GHz.